Molecular insight into the dissociation and re-formation of methane hydrate in silica nano-slit

Unveiling the nature of hydrate dissociation and re-formation in sediments is fundamental before the commercial exploitation of methane hydrates. In this work, molecular dynamics simulations were performed to investigate the dissociation process and re-formation mechanism of methane hydrate in silica-slit and bulk water. Our results indicate that methane hydrate dissociates layer-by-layer in a shrinking core manner with the generation of nanobubbles due to the supersaturation of solution. Hydrophilic quartz surface is verified to be conductive to the formation and stable existence of the ordered structures of interfacial water molecules, resulting in the survival of more residual water rings in the silica nanopore than in the bulk phase after hydrate decomposition. In the process of hydrate regeneration, it is found that intact/semi hydrate cages are verified to be beneficial for the hydrate re-formation, while the existence of large nano-bubbles induced by the long-time decomposition will be adverse to triggering the memory effect and prolong the induction time of hydrate regeneration comparing with the homogeneous supersaturated water-methane system. Moreover, the loci of hydrate re-crystallization are dominated by the concentration of dissolved methane gas in the solution and the diffusivity of water molecules. Hydrate cages are preferentially generated in the dense solution which is rich in dissolved methane gas for the unconfined water system, and the entrance of the nano-slit is usually the optimal site of hydrate reformation for the confined system due to the high concentration of dissolved gas and the appropriately restricted diffusion of water molecules. Furthermore, compared with the bulk water, a lower methane concentration is needed to trigger hydrate nucleation in silica nanopore. These findings are beneficial for a better understanding of dissociation and re-formation kinetics of hydrates at the molecular scale and provide guidelines for efficient hydrate exploitation in marine sediments.